1. Field of the Disclosure
The present invention pertains to methods for thermal treatment of printing surfaces to smooth marks resulting from development, in which the thermal treatment involves gas fluid treatment.
2. Description of Related Art
In a process for preparing from a photosensitive element a relief printing form, the printing form is often mounted around the circumference of a rotatable drum and heated to soften unpolymerized portions of the element for removal and create a relief surface of the printing form. During thermal processing, the drum is rotated and the heated printing form is placed in contact with a development medium, such as an absorbent surface, to remove the unpolymerized portions. It is desirable for thermal processing systems to heat the element to sufficient temperatures to soften or liquefy the unpolymerized portions of the element without undue heating of the element and underlying materials. In addition, development of the photopolymerizable layer produces a relief surface of recessed portions and raised portions that carry ink for subsequent printing. The raised portions should have characteristics that apply ink to print desired quality of images. While the uncured portions may be removed using a solvent to wash away the unexposed portion, this washing process requires substantial drying time. Thermal processing avoids the use of solvents and the associated time to dry the printing form.
Thermal processing of photopolymer plates should be near 250° F. for existing materials to achieve beneficial removal rates of unpolymerized portions. The temperature of the interface of the photopolymer material and the polyester base material should be less than the glass transition temperature (Tg) of the polyester base material and considerably less than temperature threshold of degradation of image registration, about 150° F. As an example of the thermal gradients required in relief printing, photopolymer layer thickness can be 0.014 inches, with 230° F. at the top surface of the photopolymer layer and 130° F. at the bottom of the photopolymer layer to avoid approaching the Tg, resulting in a temperature gradient of 7,000° F./inch (1500° C./cm). To achieve such a gradient requires rapid heating of the surface of the photopolymer layer. As can be readily appreciated, the use of only conduction from a metal or elastomer covered pressure roll through thermal insulating absorbent surface of the development medium is simply outside the bounds of such a requirement.
One example of a printing form is a relief printing plate, and in particular a flexographic printing plate. Flexographic printing plates are well known for use in printing on soft and relatively hard materials, such as packaging materials, e.g. cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive printing forms containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. DuPont has sold flexographic plates under the tradename CYREL® and has sold equipment for manufacturing the flexographic plates under the tradename CYREL® FAST.
For example, processors for thermal development of flexographic printing forms may have rotatable drums about which flexographic plates must be secured. One such processor is described in U.S. Pat. No. 5,279,697. As disclosed in U.S. Pat. No. 5,279,697, an automated process and apparatus can be used for handling an irradiated printing form and heating and pressing the print form to remove the unirradiated composition from the printing form. As disclosed in U.S. Pat. No. 5,279,697, the printing form is secured to a preheating drum with a clamp flush mounted transversely on an outer surface of the drum.
Similarly, in a commercial thermal processor sold under the tradename CYREL® FAST 1000TD, an exterior surface of the drum includes a silicone rubber layer having a Shore A hardness of 50, and a tackification layer on the rubber layer. The tackification layer is DOW 236, a silicone dispersion in solvent, available from Dow Corning, and it adheres the printing form to the drum.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 and U.S. Pat. No. 6,797,454 each describe an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element with a web of absorbent material. Both apparatuses include a hot roller for supporting the web of absorbent material in contact with the photosensitive element to heat the element and remove the molten polymer with the absorbent material during thermal development. In U.S. Pat. No. 5,279,697 the element having irradiated and unirradiated areas is mounted on a preheating drum that is heated with an electrically heated blanket mounted on an inner surface of a main wall of the drum. The heat must travel through the wall of the drum and through the flexible substrate to preheat the composition layer to a temperature near the melt point of the unirradiated area. But the heated drum may heat the support of the element to such an extent that the support may distort and/or shrink which can affect the dimensional stability of the relief area of the printing plate. U.S. Pat. No. 6,797,454 avoids such undesirable distortion and/or shrinkage of the support by cooling the support simultaneous to the heating of the composition layer. Cooling of the support is carried out by blowing air on the circumferential surface of the drum while providing additional heat to an exterior surface of the composition layer with infrared heating devices prior to contact of the absorbent material with the hot roller.
U.S. Pat. No. 4,197,126 discloses an air etching process using a gas stream. Materials removed via the gas stream are relatively thin liquids having material viscosities of 100-10,000 cps between room temperature and 200° F. (93° C.), preferred material viscosity is 200 cps.
It is desirable for thermal processing systems to heat a photopolymerizable element to sufficient temperature to soften or liquefy unpolymerized portions of the element without undue heating of the element, and in particular without overheating a polymeric film support that underlies the layer of the photopolymerizable composition.
However with increasing demands on quality, the current state-of-the-art flexographic printing forms may not perform as desired and have trouble meeting the ever increasing demands on quality. Another problem that may arise from thermal processing is that contact of the absorbent material or development medium to the photopolymerizable layer can induce patterns or marks on uppermost surfaces of the raised portions, i.e., printing surfaces of the relief. The patterns, sometimes referred to a web patterning or web marks, of the resulting relief printing form induce a matte-like appearance to the printing form. More importantly, in some cases the pattern or marks can be observed in the image printed on the substrate. Further, sometimes fine raised elements of the relief surface become bent or deformed from pressure contact of the development medium supported by the heated roll during thermal development.
So a need arises to be able to prepare a relief printing form in which an artefact, marking or patterning induced from the development medium as well as the bending of fine raised elements of the relief surface that are induced by pressure contact of a heated roll associated thermal development is reduced or eliminated to provide suitable appearance of the printing form and desired print quality.